1. Field of the Invention
The present invention is directed to an electrical multilayer component which may be a capacitor, a PTC. resistor or a varistor.
2. Description of the Prior Art
Electrical multilayer capacitors having a sintered, monolithic ceramic body of dielectric material of the type to which the present invention is directed has already been disclosed in patents in various embodiments. Of the extensive patent literature U.S. Pat. Nos. 3,679,950; 4,071,880; and 4,658,328 are believed to be the most pertinent.
U.S. Pat. No. 3,679,950 discloses a ceramic multilayer capacitor which includes cavities which alternate from layer to layer of the multilayer structure, but are only open toward the end faces which are in opposed relation but are not open toward the adjacent lateral surfaces which are at right angles to the end faces. This U.S. patent also discloses a method for the manufacture of an electrical multilayer capacitor wherein a suspension of particles is printed on to the ceramic body in limited region to produce cavities which are open only to one side. Since the finished stack of a plurality of layers prepared in this way is closed on practically all sides, it is necessary that the stack be separated into discrete members along parting lines that exist perpendicular relative to one another so that the escape and/or the decomposition of volatile constituents to be removed from the suspension for the formation of the cavities can occur in a subsequent sintering process.
In addition to other possibilities, a metal or alloy having a low melting point, for example, lead or an alloy composed of 50% Bi, 25% Pb, 12.5% Sn and 12.5 Cd are mentioned for the manufacture of the capacitor coatings inside the cavities of the monolithic ceramic body. While these alloys have melting temperatures which are considerably lower than the sintering temperature used for sintering the monolithic ceramic body, they do not wet the surface of the ceramic body in the cavities, or wet it only poorly.
With the multilayer capacitor disclosed in U.S. Pat. No. 3,679,950 the contact layers which are usually composed of silver are always applied to the end faces after a molten metal having a low melting point has been pressed in and after the cooling of the saturated ceramic body.
U.S. Pat. No. 4,071,880 discloses practically the same electrical multilayer capacitor comprising a sintered, monolithic ceramic body as that disclosed by U.S. Pat. No. 3,679,950. Beyond this, U.S. Pat. No. 4,071,880 also discloses the possibility of applying the contact layers to the end faces of the monolithic ceramic body before the introduction of metal, for which purpose these contact layers must be made porous. The effect is that molten metal can not flow out after removal from the melt because the metals employed do not wet the ceramic body or wet it only poorly.
The multilayer capacitor disclosed in U.S. Pat. No. 4,071,880 is composed of a plurality of ceramic layers and internal electrodes which lie on top of one another in alternating fashion and is also composed of outer, double layer electrodes which are connected to the inside electrodes in a predetermined fashion. For example, a comb-like electrode structure may be present.
In the manufacture of such a monolithic multilayer capacitor, untreated or unsintered ceramic laminae are first produced with the assistance of a stripping or squeege method using a stripper or scraper blade. These ceramic laminae have a thickness of 0.05 to 0.1 mm. A carbon containing paste is then applied to or printed on to the surfaces of the ceramic laminae, the paste being composed of a mixture of carbon particles and ceramic powder. This paste is applied in limited regions, such that the cavities in the finished ceramic body are alternately opened only toward the opposite end faces. A plurality of such printed ceramic laminae are arranged on top of one another alternately and are joined to one another for forming an integrated structure by applying pressure. Discrete bodies are produced from this integrated structure by cutting along lines that are located perpendicularly relative to one another, the discrete bodies being sintered at a temperature above 1,000.degree. C. in order to sinter the ceramic laminae and in order to eliminate the carbon powder within the paste that has been printed on. As a result, porous intermediate layers comprising ceramic powder are produced in regions in which the inside electrodes are to be formed. Following this, the porous outer electrodes or contact layers are applied to the sintered discrete bodies.
In the case of the prior art as in the case of the present invention, the electrodes can be applied with methods that are disclosed in U.S. Pat. Nos. 3,683,849; 4,526,129; and 4,561,954; in British patent No. 2,106,714; in German patent No. 27 22 140 or in German published application No. 36 38 286.
For example, the outer electrodes may be obtained by firing a paste that is principally composed of nickel and is mixed with a glazing compound. A ceramic body formed in this way is then introduced into a pressure vessel and is dipped into molten lead that serves as the conducting metal for the capacitor coatings. The temperature of the melt is about 330.degree. to 360.degree. C. in its unpressurized condition. Subsequently, the pressure is elevated to about 10 bar so that molten lead penetrates into the cavities of the ceramic body under pressure, passing exclusively through the porous, outer electrodes. The ceramic body is then removed from the molten lead, cooled, and exposed to normal pressure so that the inner electrodes of lead are formed. Following this, additional layers that ar solderable are applied to the outer electrodes.
In order to manufacture the described multilayer capacitor, the outer electrodes applied to the monolithic ceramic body must be porous and must be manufactured such that the entry of molten lead is initially possible for the inside electrodes but lead is prevented from flowing out of the cavities when the ceramic body is removed from the molten lead. As recommended in U.S. Pat. No. 4,071,880 the metal to be introduced should not easily wet the ceramic body. In other words, the porous, outer electrodes form penetration barriers under certain conditions. The porous, outer electrodes are principally composed of nickel that does not react with lead. The adhesion to the end faces of the ceramic body depends on the amount of glazing compound contained within the paste, the adhesion being improved with an increasing amount of glazing compound. In this case, however, the number of pores in the outer electrode is diminished, so that the penetration of the lead is rendered difficult. The electrostatic capacitance therefore can not be controlled in a desired way, even when the ceramic module is exposed to a relatively high pressure within the melt.
In order to overcome the described problems, U.S. Pat. No. 4,658,328 corresponding to German published application No. 36 12 084 has proposed that the cavities in the interior of the ceramic body be designed such that they are open both to end faces which oppose one another as well as, to a slight degree, toward the adjoining lateral surfaces so that the molten metal during the injection process can penetrate not only through the porous contact layers but also penetrate through the lateral surfaces that are open to a slight extent and through parts of the cavities that are free of contact layers but can nevertheless not flow out from the cavities This U.S. patent also discloses a method corresponding to that previously described but with the limitation that the contact layers applied to the end faces have to be porous.
German Published Application Nos. 36 27 936 and 36 27 929 disclose multilayer capacitors comprising a sintered, monolithic ceramic body and methods for the manufacture thereof similar to those already described. In contrast with previous disclosures, however, these published applications propose that a metal that wets the surface parts inside the cavities of the ceramic body be used as an easily melting metal for the capacitor coatings.
According to these published German applications, pressing metals into the ceramic bodies continues to occur through the openings of the cavities that are directed toward only one side and which may be free of a contact layer or covered by a porous contact layer. In the former case, the contact layers are subsequently applied.
The use of a metal that wets the surface of the ceramic body has the advantage that the metal no longer flows out the cavities of the ceramic body when it is removed from the metal melt and that a good bonding of the capacitor coatings to the contact layers is guaranteed.
When the easily melting metal is pressed through porous contact layers into the cavities in the ceramic body, the cavities being opened only toward one side, the problems that have already been discussed appear, namely, the risk of blocking the pores in the porous layer and, thus, the risk of an inadequate filling of the cavities. These problems are not eliminated even when employing metals which wet the ceramic well. Problems also arise when applying contact layers following the impregnation with the metal melt and cooling of the filled ceramic bodies, in that the bonding of the capacitor coatings in the inside of the ceramic body through the contact layers at the end faces can be inadequate.
U.S. Pat. No. 4,584,629 corresponding to British patent No. 2 162 371 or German Published Application No. 35 09 593 contains a very detailed description of the manufacture of multilayer capacitors. With these capacitors, too, the contact layers are applied to the opposite end faces of the sintered ceramic bodies before the metal is put into the cavities. The technique set forth therein, as well as in the other patents, can likewise be employed for manufacturing the ceramic body and for pressing the metal melt into the cavities in the present invention.
Ceramic PTC resistors in layer form are disclosed, for example, in British Patent No. 932 558 and ceramic multilayer varistors are disclosed, for example, in U.S. Pat. No. 4,675,644 correspondig to EP-A-No. 0 189 087. In these disclosures, the metal coatings are already produced before the sintering of the ceramic bodies, using heat resistant precious metals such as platinum, palladium, silver or the like, as likewise disclosed in U.S. Pat. No. 3,740,624.